Common Language Runtime

Question: What services does the .NET common language runtime provide on top of the services that are offered by Windows? What are the benefits of these services?

Answer: The common language runtime (CLR) offers the following services on top of the Windows operating system (or an alternative operating system):

• Language interoperability. This means binary compatibility and seamless cooperation between programs written in any of more than 20 .NET languages. All .NET programs are translated to CIL code, which is compiled to machine code at load time (just in time). The common type system (CTS) defines the types that are supported by the CLR. A subset of these types is called the common language specification (CLS). These are the types that have to be supported by all languages that wish to cooperate with others under .NET. The CLS also makes sure that subclassing, threading and exception handling work the same way in all of these languages and can thus be transparently used across all languages.



• Garbage collection. The CLR has a garbage collector that makes sure that all dynamically allocated objects are automatically reclaimed as soon as they are not referenced any more. This avoids a lot of common programming errors such as deallocation an object that is still in use or forgetting to deallocate an unused object.



• Versioning. .NET programs come in components that are either stored in EXE files or in DLL files. These components are self-contained, i.e. they contain information about all referenced components as well as type information, which can be used for reflection. There is no need for making registry entries for assemblies, because they contain all the information that is necessary to load and execute them. Deployment can therefore be as simple as copying the DLL and EXE files to the disk.
  An assembly is not only referenced by its name but also by its version number. The compiler stores the version number of referenced assemblies in the object file of the client program. When this client is loaded, the loader searches for the required assemblies with the version numbers that were seen at compilation time. This guarantees that the type checks of the compiler are still valid at run time. Components with the same name but with different version numbers can co-exist on the disk as well as in main memory. Installing new software can therefore never destroy identically named DLLs of some other application. This is called the end of the DLL hell.



• Security. When an assembly is loaded it is first verified to ensure that its object code has not been manipulated to break the type rules. Assemblies can also be signed using public key encryption. This guarantees that an assembly really comes from a certain trusted institution instead of from a hacker. For every assembly as well as for every zone or publisher the administrator can define code-based as well as role-based rights. This can be used to run untrusted applications in a sandbox instead on an open machine.